Information on many different types of organic light emitting diodes (LED) has been published. In general these prior art devices suffer from low efficiency, relatively high drive voltages, poor reliability and/or limitations in manufacturing capabilities (poor adaptability).
Many of these prior art devices use a low work function metal, such as MgAg (magnesium, silver) for an electron injection contact. Other devices utilize a calcium electron injection contact. A layer of electroluminescent organic material, such as Alq.sub.3 (8-hydroxyquinoline aluminum), is applied to the electron injection contact and a hole injection material such as [1, 1-bis(4-di-p-tolylaminophenyl cyclohexane (BTC)] is applied to the Alq.sub.3 to produce a PIN diode junction. The MgAg, or calcium, injects electrons into the junction, which electrons tunnel into the Alq.sub.3 where they recombine with holes from the [1, 1-bis(4-di-p-tolylaminophenyl cyclohexane (BTC)] to form light.
The prior art discusses minimizing the Schottky barrier height so that the tunneling distance is reduced and, in some cases may even be eliminated. This decreases the work function of the cathode (electron injection contact) by decreasing the barrier for electron injection from the cathode to the organic recombination layer (electroluminescent organic material). This decreases the forward voltage of the device, thereby, allegedly increasing the net device efficiency. However, it should be noted that there is no, or very little, barrier for holes to flow from the electroluminescent organic material into the low work function cathode metal. Thus, metal based low work function tunneling injection cathodes suffer from a low injection efficiency. In other words, holes may flow into the metal cathode freely using up the applied current and creating heat.
Most of the prior art devices utilizing low work function cathodes utilize magnesium or calcium as the low work function material. A major problem with these devices is that the low work function metals are oxidation sensitive, leading to reliability problems with the cathode. These reliability problems are widely reported in the literature.